Prior art sewage grinder pumps, such as disclosed in U.S. Pat. No. 4,014,475 issued Mar. 29, 1977, for a "Combined Manway and Collection Tank for Sewage Grinder", Richard C. Grace et al inventors, assigned to Environment/One Corporation, employs an inverted diving bell-type airtight and liquid-tight enclosure for supporting a pressurized air column that operates a pneumatic/electric switch to control the normal on/off operating levels of the sewage grinder pump (sgp). A separate alarm overflow level sensing and alarm indicating pneumatic/electric switch also is included in the sgp system. This known control arrangement is satisfactory in many respects for operating the sewage grinder pump (sgp) but requires that one side of the pneumatic/electric switches be vented to atmosphere to prevent false indications of water level due to ambient pressure changes and/or unbalanced pressures within the collection tank containing the sgp. This vent normally has taken the form of a flexible, small diameter, plastic tube, the entrance to which is located high up in the tankage and is subjected to ambient pressures. Even when proper venting is obtained, because the collection tank presently used is comprised of a sealed basin and a vented accessway as disclosed in Pat. No. 4,014,475, rapid pressure variations in the sealed basin portion of the collection tank can cause false indications. To overcome this problem a timed delay relay is required in the starting/relay circuitry for the sgp motor. These features add complications to the design of the sewage grinder pump system, its controls, and the collection tank, and often lead to a rather messy installation which leaves electric conductors and venting tubes hanging loose within the collection tank.
To overcome these problems, the present invention was devised.